"Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men."
This arose as a quotation by Lord Acton in a letter to Bishop Mandell Creighton in 1887.
William Pitt the Younger, The Earl of Chatham and British Prime Minister from 1766 to 1778, is sometimes wrongly attributed as the source. He did say something similar though, in a speech to the UK House of Lords in 1770:"
Lord acton was a british historian and as a historian, he had insight into these matters, the fact that he was catholic was also significant.